This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-333581, filed Oct. 31, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a track-following method in an optical disc apparatus and an optical disc apparatus that adopts this method and, more particularly, to a method of moving an optical pickup to seek a target track and guiding a light beam spot to the center of the track upon tracking the target track on an optical disc in an optical disc apparatus that can record data on an optical disc such as a DVD-RAM disc which has a header field having staggered pre-pits (so-called CAPA), and an optical disc apparatus that adopts this method.
2. Description of the Related Art
In recent years, specifications of a DVD-RAM (Digital Versatile Disc of Random Access Memory Type) disc as an optical disc have been provided. This DVD-RAM disc is provided with a phase-change type recording layer that allows overwrite, and adopts a single-track format. In the single-track format, data can be recorded not only on grooves but also on lands between neighboring grooves for the purpose of increasing the recording capacity, and in which a land and groove alternately appear per round. Therefore, since a recording track continues from a land to a groove, and then from a groove to a land, data can be continuously recorded. Since the DVD-RAM disc adopts a scheme for recording marks on lands and grooves, crosstalk is suppressed by adjusting the groove depth. That is, each groove is formed to have a predetermined depth (60 to 70 nm) so as to obtain an identical reflection intensity from a track independently of the presence/absence of marks on a neighboring track.
Furthermore, in the DVD-RAM disc, the track is segmented into a plurality of sectors, each of which has a header field, data recording field, and mirror fields located between these two fields. The header field located at the head of a sector has an embossed header formed by pre-pits, where fields PID1 to PID4 are formed by writing ID (identification data) four times. The fields PID1 and PID2, and fields PID3 and PID4, are offset to the left and right with respect to the center of the track. The left and right offset fields PID1 and PID2 and fields PID3 and PID4 are called CAPA (Complementary Allocated Pit Address).
It is very important for an optical disc apparatus that drives an optical disc such as the DVD-RAM disc to improve seek performance for seeking a target track. The optical disc apparatus has a track access mechanism for accessing a target track, and this track access mechanism normally uses two stages of mechanisms, i.e., coarse movement and fine movement. A seek sequence is executed in the order of coarse seek, center guide 1, fine seek, and center guide 2. Note that the track-following process is a control process for setting the relative speed between the disc and light beam spot formed on the disc in the radial direction of the disc to be zero, and setting the beam spot coming from an objective lens at the track central position, and relative motion between the disc and light beam spot is detected by a tracking error signal (T SUB signal) which is also called a tracking error signal (TE signal).
It is not so easy to realize a stable track-following process of the objective lens since the track-following process depends on the actuator sensitivity of a lens fine movement system, and the tracking error (TE) signal has nonlinear periodicity. For example, even when tracking-servo is suddenly started in the vicinity of a target track, since the deceleration performance of a lens actuator is limited, a track-following failure such as xe2x80x9ctracking-servo failurexe2x80x9d (the lens fine movement system repeats acceleration/deceleration) may occur.
As a method of realizing a stable track-following process, various devises are proposed. For example, in a DVD-ROM drive or the like, a so-called xe2x80x9chysteresis track-followingxe2x80x9d scheme for executing a hysteresis process of a tracking error (TE) signal on the basis of a track sum signal having a 90xc2x0 phase difference from the tracking error (TE) signal to attain a stable track-following process is known, and this scheme is adopted.
Since the DVD-RAM disc adopts land-and-groove recording, the groove depth is determined to obtain equal amounts of light reflected from the land and groove and, hence, a sum signal having a 90xc2x0 phase difference from the tracking error (TE) signal cannot be generated. As a result, the xe2x80x9chysteresis track-followingxe2x80x9d scheme cannot be applied to the DVD-RAM disc.
The DVD-RAM disc has pre-pits called CAPA, as described above, and upon detecting the CAPA before the end of track-following, noise is produced in the tracking error (TE) signal due to the CAPA, and the track-following process of the DVD-RAM disc drive becomes unstable.
In the conventional track-following process of the DVD-RAM disc drive, the control waits until the relative speed between the disc and lens becomes sufficiently small, e.g., until it is confirmed that the relative speed becomes 7 mm/s or less, and then switches to tracking-servo. However, the wait time required until the relative speed between the disc and light beam spot becomes sufficiently small disturbs a decrease in seek time.
As a processing method that allows to guide the beam spot to the center of a track even when the relative speed between the disc and light beam spot becomes higher than the current upper limit, a method of decelerating the lens using brake pulses may be used. In the brake pulse deceleration method, when the phase of a tracking error signal has been reached, maximum pulses are supplied to the lens fine movement driving system for a predetermined period of time so as to reduce the relative speed, thus achieving a nearly zero relative speed state near the center of a track within the shortest time, and tracking-servo is then started. As the control system, since the relative speed is abruptly decreased by an open lens actuator command, the track-following process can be done even when the relative speed is high at the beginning of tracking-servo. In this case, if relative motion (relative acceleration, relative speed, relative position) is constant at the beginning of tracking-servo, no problem is posed. However, if relative motion is not constant at the beginning of the track-following process, the light beam spot cannot be stably guided to the center of a track.
FIGS. 1A and 1B show the time responses of a track-following command to a lens actuator and a tracking error (TE) signal upon guiding a light beam spot to the center of a given track by the method of decelerating the lens using brake pulses. In FIGS. 1A and 1B, reference symbol I denotes a tracking error (TE) signal; and II, an output (DSPout) from a digital signal processor (DSP). FIG. 1A shows a case wherein the initial thrust speed of the lens is 14 mm/s, and the acceleration is zero (0 m/S2). In this example, brake pulses are generated before an elapse of time xe2x80x9c0.3 msxe2x80x9d, and tracking-servo is then started to make the tracking error (TE) signal converge to zero. In the example shown in FIG. 1A, the light beam spot is reliably guided to the center of the target track. By contrast, FIG. 1B shows a case wherein the initial thrust speed of the lens is 9 mm/s smaller than 14 mm/s, and the acceleration is zero (0 m/S2). In this example, brake pulses are generated after an elapse of time xe2x80x9c0.25 msxe2x80x9d, and tracking-servo is then started. However, the tracking error (TE) signal cannot be converged to zero, and the light beam spot cannot be guided to the center of the target track, thus producing so-called xe2x80x9ctracking-servo failurexe2x80x9d.
As can be seen from FIGS. 1A and 1B, the method of decelerating the lens by brake pulses can successfully guide the light beam spot to the center of a track but may cause xe2x80x9ctracking-servo failurexe2x80x9d depending on the speed at the beginning of tracking-servo. That is, the method of decelerating the lens by brake pulses has a narrow allowable range of the initial thrust speed, and thrust at low speed falling outside this allowable range causes xe2x80x9ctracking-servo failurexe2x80x9d. Although not shown, even within a range that allows a track-following process, if a light beam spot crosses CAPA immediately after tracking-servo is started, the servo system becomes unstable, and the track-following process fails. In any case, a sufficiently stable track-following process cannot be realized by only the brake pulse deceleration method.
As described above, in the track-following process in the conventional DVD-RAM disc drive, the track-following process must be started after the relative speed between the disc and light beam spot becomes considerably low so as to attain a stable track-following process, and such wait time disturbs a decrease in seek time. Even by the brake pulse deceleration method that can attain a track-following process even at a high relative speed between the disc and light beam spot, a sufficiently stable track-following process cannot be realized.
It is an object of the present invention to provide a track-following method in an optical disc apparatus that can stably guide a light beam spot to the center of a target track even when an objective lens crosses a track at a higher relative speed in an optical disc apparatus for driving a disc having CAPA such as a DVD-RAM disc, and an optical disc apparatus which adopts this method.
According to an aspect of the present invention, there is provided a method of guiding an optical beam on a target track on an optical disc having lands and grooves defining tracks, each of the tracks including sectors having header regions in which pre-pits are formed, comprising:
moving the light beam in a direction to cross the tracks to seek the target track on the optical disc;
detecting the light beam reflected from the optical disc to generate a tracking error signal having zero-crossing points;
detecting the pre-pits from the tracking error signal, and detecting a predetermined one of the zero-crossing points after the detection of the pre-pits to generate a start signal; and
starting tracking-servo in response to the start signal to guide the optical beam on the target track in accordance with the tracking error signal.
According to an another aspect of the present invention, there is provided an optical disc apparatus configured to guide an optical beam on a target track on an optical disc having lands and grooves defining tracks, each of the tracks including sectors having header regions in which pre-pits are formed comprising:
a focusing lens configured to focus the light beam on the optical disc;
moving mechanism configured to move the focusing lens and shift the light beam in a direction to cross the tracks to seek a target track on an optical disc;
detecting circuit configured to detect the light beam reflected from the optical disc to generate a tracking error signal having zero-crossing points;
pre-pit detecting circuit configure to detect the pre-pits from the tracking error signal, and detecting a predetermined one of the zero-crossing points after the detection of the pre-pits to generate a start signal; and
track-servo controller configured to start tracking-servo in response to the start signal to guide the optical beam on the target track in accordance with the tracking error signal.